This invention relates to speed sensors of the kind that produce a pulse signal with a repetition frequency related to the speed being sensed. In particular, the invention relates to the use of speed sensors of the aforesaid kind in anti-lock brake control systems for vehicles with braked wheels in which the sensors are used to sense wheel speed.
Anti-lock brake control systems detect impending wheel lock conditions by sensing the corresponding fall in wheel speed and respond by releasing the brakes so that wheel speed can recover. In high performance systems, the wheel speed sensors employed respond to a rapid fall in wheel speed such as occurs during two or three pulses of the sensor pulse signal. However, this high sensitivity can then lead to the dangerous situation in which cyclic malfunctions in the sensor are interpreted as a fall in wheel speed and the brakes are released. For example, if a sensor has a rotor with a series of teeth that produce the pulses, and one or more teeth is distorted, broken or missing due to damage caused by the intrusion of a foreign body or excessive rotor run out or bearing wear, one or more pulses will be missing per revolution thereby producing a sensor signal with cyclic variations that can cause repeated brake release, possibly leading to a complete loss of brake pressure.